Radio receiving system



May 28, 1935. P. OUFARNHAM RADIO RECEIVING SYSTEM Filed Nov. 4, 1952 F1 B. l-

Patented May 28, 1935 'QUNITED STATES.

RADIO RECEIVING SYSTEM y Paul 0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, Delaware N; Y., a corporation of Application November 4, 1932, Serial N0.'641,2 80

7 Claims.

This invention relates to radio receiving systems and more particularly to systems of the automatic gain control type.

It has been found desirable in the high gain receivers of this type to provide an additional automatic control which operates to cut off all receiver output in the absence of a radio signal strong enough to override the prevailing noise level of the locality where the receiver is operated.

Methods and circuit arrangements for accomplishing this general result are described and claimed in my copending applications Ser. No. 524,383, filed March 21, 1931, Ser. No. 585,593, filed January 8, 1932, Ser. No. 603,849, filed April '7, 1932, and Ser. No. 641,278, filed November 4, 1932.

In accordance with the inventions described in the three later cases, the detector and/ or audio amplifier are normally biased to suppress transmission of signals and a direct current amplifier or relay tube is provided to remove the transmission suppression bias when the strength of the received carrier wave risesabove a predetermined level, the direct current voltage impressed on the relay tube being obtained by-rectification of the amplified carrier wave voltage.

An object of the present invention is to provide in a radio receiver system, improved methods of and circuit arrangements for the automatic control of an operating characteristic of the re ceiver system in accordance with the magnitude of the radio input to the system. An'object is to provide, in a radio receiver, improved methods of and circuit arrangements for suppressing all receiver output when the amplitude of the carrier wave to which the receiver is tuned falls below a predetermined value. A further object is to provide a radio receiver system'in which a single tube may perform the functions of the direct purrent amplifier or relay tube of a noise suppression system and also may serve as the detector and as a source of automatic gain control bias. A further object of the invention is to provide a noise suppression system for radio receivers which does not affect the receiver operation so long as the received radio signal .is above a predetermined magnitude, and which provides a sharp cut off for all receiver output when the received radio signal falls below that value.

These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawing in which-Figs. 1 and 2 are fragmentary circuit diagrams of embodiments of the invention In the drawing, the reference character I identifies a radio frequency amplifienincluding one or more vacuumtubes, which receives car rier Wave energy from an appropriate collector structure and which works into. a detector and audio amplifier or other load circuit. The exact design and construction of the radio amplifier are not a part of the present invention, and only the last amplifier tube 2 is illustrated. I

The tuned output circuit .3 of the last amplifier stage is coupled to the tuned input circuit 4 of the detector and automatic control stage of the receiver, the radio: voltage E0 which is built up acrossthe primary circuit 3 establishing a radio voltage E across the secondary circuit l. The single tube 5"of the combination detector-control stage may be a tube known as Type-55, and includes a pair of rectifying anodes A1, A2, a control grid G, plate P and a cathode K.

Referring first to the circuit elements which cooperate with the tube 5 to give the stage the function of a detector and an audio frequency amplifier, the high potential terminal ofthe tuned input circuit 4 is directly connected to the the control grid G through a filter comprising resistance 9 and a radio frequency condenser Ill. The plate circuit includes the primary winding of an audio frequency transformer ll, preferably provided with means such as a shunt resistance l2 and adjustable tap l3 for controlling the.

output level of the receiver system, and a source of direct current potential +B which is connected to ground or -B through the voltage. divider formed by resistances l4, IS. The cathode. K is connected to ground through a resistor I 6 which is by-passed for audio frequency'bythe condenser H. A direct current path for impressing a bias voltage between the. anode'Az and the cathodeK is completed by the resistance [8 which is connected between the resistance 6 and the junction of, the resistances l4, l5, of the voltage divider.

The flow of plate current through the cathode resistor IE will establish a potentialv drop E1 which places the cathode K at a more positive direct current potential than that of grounder B. The direct current voltage E2 across the voltage divider l4, l5 gives rise to a potential E: across resistance M which applies a positive potential to the anode A2.

The bias voltage on the anode A2 is equal to the difference of the voltages E1 and E3. The voltage E3 and the value ofresistance l6 are so chosen'that, when the input voltage E is zero, a sufficient plate current will fiow in the tube 5 to make the voltage E1 greater than the fixed voltage E3. In other words, the plate current has a normal value in the absence of received signals which is sufficient to maintain the anode A2 and the control grid G at direct current potentials which are negative with respect'to the cathode. It therefore follows that small peak values of E lessthan E1E3 will not be rectified in the anode Az-cathode circuit and hence no audio frequency voltage will be developed across the resistor 6.

' Attention is directed 'to the fact that the connection betweenthe audio frequency resistance 6 and the grid G will impress both the rectified audio voltage and the rectified direct current voltage upon the grid G, thus causing the tube 5 to function as an amplifier for any direct current voltage developed across resistance 6. This amplified direct current voltage operates to reduce or remove the bias voltages normally in:-

pressed upon the anode A2 and the grid G. When the peak value of the radio input voltage E exceeds the bias voltage E1-E3 on the anode A2, rectification takes place in the circuit including the elements A2, G and an additional negative bias is thus impressed upon the control .grid, which bias effects a reduction in the plate cur rent flow. The consequent reduction in the voltage E1 across resistance l6 removes the initial biasfEiEs on the anode A2, and the normal diode detection then takes place in that circuit. The direct current voltage developed across resistance 6 by rectification, together with the de crease in the voltage E1 across the cathode resistance l8 serves to maintain the control grid bias at a value suitable for audio frequency amplification. v

A bias voltage for. automatically controlling the gain of amplifier I is developed between anode A1 and cathode K by connecting anode A1 to the tuned primary circuit 3 through a radio frequency condenser l9 andto'ground through a resistance 29. The anode terminal of resistance 20 is connected through a filter comprising resistance 2| and condenser 22 to the lead 23 which extends to the control grid circuits of the tube or tubes of amplifier l.

It is desirable that the magnitudes of the radio voltages, E and E0, be so related that when the detector input E is increased tothat value sufiicient to make the voltage drop 2E1 across, the cathode resistance l6 equal to the fixed bias component E3, the peak voltage E on the auto- -matic gain control anode will be equal to E1, and

thus initiate the action of the automatic gain control. To make the voltage E0 greater than E, the inductance of the primary tuned circuit 3 may be larger than that of the secondary circuit 4.

The operation of the circuit is as follows. So long as the'peak detector input voltage E is less than the negative bias voltage, E1 E3, on the anode A2, there will be no rectification of the impressed voltage E by the diode A2K. Nor will there be any incidental rectification of radio 'voltagesin the triode amplifier circuitsince the voltage E1.

developed across resistance 6 are impressed on the grid G. The rectified direct current voltage increases the negative bias on grid G, thus reducing the plate current and thereby reducing the This reduces the delay action bias voltage on anode A2, and reduces the negative bias on grid G to a value appropriate for audio amplification.

Turning now to the automatic gain control, the delay action bias voltage on anode A1 is aqual to E1. As soon as rectification takes place in the detector diode circuit, the voltage Ei'is'reduced by the direct current amplifier action of the triode elements and the delay bias is thus removed from the automatic gain control diode. The rate at which a gain control voltage is developed, with increasing radio input voltage E0, is therefore more rapid than is the case when a fixed delay bias voltage is impressed upon the gain control rectifier. This method of and circuit arrangements for removing the delay bias impressed on a gain control rectifier are described and claimed in my copending application Ser. No. 641,279, filed November 4, 1932.

It will be noted that the tuned circuit 4 provides greater selectivity in the transmission of ampliselectivity of transmission to the rectifier Ai K.

creases more rapidly than the radio input E0 to the rectifier when the amplifier l is detuned from resonance at the frequency of a received carrier wave. The audio output falls ofi rapidly when the receiver is detuned and the actual selectivity of the receiver is not materially affected by the automatic gain control. The previous types of gain control systems did not exhibit this difierence in the selectivity of transmission to the rectifier and to the detector, and theamplifier gain therefore increased at a rate which approximately compensated for the decrease in radio input to the detector. The response from local stations was thus maintained approximately constant over a wide tuning range as detuning from a strong signal resulted in an automatic increase in sensitivity which counteracted the decrease in audio output which would normally follow from the selectivity characteristics of the tuned amplifier. The differential transmission obtained by means of the Fig. 1 circuit prevents the gain from rising at such a rate as to destroy the selectivity of the amplifier.

The detector-control stage illustrated in Fig. 2 functions in the same manner and those circuit elements which are, or may be, identical with those described above are identified by like reference numerals. The input to both diodes is derived from the tuned secondary circuit 4, the input E0 to the anode A1 being taken off at higher voltage point, through condenser l9, than that of the input voltage E to anode A2. As described and claimed in the last named application, the audio output from tube may be taken across the resistance [6, or between the grid G and ground since the audio voltage across the resistance is in The radio input E to the detector therefore de a elements were used in an embodiment of the Fig.

1 circuit which was employed as the second detector of a superheterodyne receiver.

Resistance. 6=2G0,G00 ohms Resistance 9=l00,000 ohms Resistance l6=15,000 ohms Resistance i8=59,000 ohms Resistance 28:1 megohm Resistance 2 I =500,900 ohms Condensers 1, I1=0.5 microfarads Condensers 8, ill, I 9:100 micromicrofarads Condenser 22:0.05 microfarads Condenser E2=250 volts Condenser E3=60 volts The inductance of circuit 3 was 18 millihenries and that of the secondary circuit 4 was 4.5 millihenries. I

The invention is not limited to the use of the relative values of this particular example, nor, in its broader aspects, to the use of a single tube in the combination detector-control-amplifier stage.

I claim:

1. In a radio receiver, an automatic control system comprising a pair of rectifiers, input means for impressing radioyvoltages upon each rectifier, an output circuit for each rectifier, means impressing bias voltages upon each rectifier to render the same inoperative for radio input voltages of less than predetermined values, and means energized by the rectified output of one rectifier for automatically reducing both bias voltages when the input on said one rectifier rises above the predetermined value below which no rectification obtains.

2. In a radio receiver, a diode detector, a diode rectifier forming part of an automatic transmission control system for said receiver, an amplifier for amplifying the rectified direct and alternating current voltages developed by said detector, a resistance in the plate circuit of said amplifier, means including said resistance for impressing upon said detector and rectifier delay bias voltages which vary with the plate current fiow, the bias voltage initially impressed on the detector being of a magnitude to prevent rectification for all detector input voltages below a predetermined value, and means for impressing on said rectifier a radio input voltage less than and proportional to the radio input voltage on the detector.

3. In a radio receiver, the combination with a tube having two anodes, a control grid, a plate and a cathode, of a radio frequency amplifier and circuit elements coupling said amplifier to,

said anodes and cathode to form radio input circuits of a diode detector and a diode rectifier,

an output circuit forsaid diode detector and coupled to said amplifier to impress thereon the rectified direct and alternating current voltages developed by said detector, said output circuit including a resistance across which the flow of amplifier plate current establishes a potential drop that tends to place a negative bias on the detector anode, an output circuit for said diode rectifier, means coupling the rectifier output cir.

cuit to said radio frequency amplifier to apply thereto a bias voltage, and means for biasing the rectifier anode by the direct current potential drop across said plate circuit resistance.

4. In a radio receiver, the combination with a radio amplifier, and a detector, of means including a diode rectifier for automatically controlling the gain of said amplifier as a function of the strength of received signals, an input circuit for impressing upon said .diode rectifier a radio voltage developed by said radio amplifier, means initially impressing upon said diode rectifier a bias voltage which renders the same inoperative for impressed radio voltages having voltage peaks of less magnitude than said impressed bias volt-' age, and means energized by a rectified voltage passed by said detector and operative when the rectified voltage exceeds a predetermined value to remove from said rectifier the bias voltage which renders the same inoperative.

5. In a radio receiver, a radio frequency amplifier, a detector working out of said amplifier, means biasing said detector to render the same inoperative for radio input voltages below a critical value, normally inoperative means including a rectifier working out of said amplifier for automatically controlling the gain of said amplifier, and. means energized by a rectified voltage developed by said detector for rendering said automatic control means operative.

6. The invention as set forth in claim 5, wherein the rectifier of said automatic control means is normally biased to render the same inoperative, and said last means comprises an impedance across which a potential drop efiective to remove said normal bias is established when said detector passes a rectified voltage of predetermined magnitude.

7. In a radio receiver, the combination with a radio amplifier, a detector, a direct current amplifier working out of said detector, and a resistance in the output circuit of said direct current amplifier, of a diode rectifier having a radio input circuit coupled to said radio amplifier, said resistance being included in its input circuit to impress thereon a bias voltage which varies with the output of said detector, a second resistance in the output circuit of said diode rectifier, and a circuit connection for impressing the variations of direct current potential developed across said second resistance upon said radio amplifier as a gain control bias.

PAUL O. EARNHAM. 

